1. Field of the Invention
This invention relates to a tracking controlling method and apparatus employing push-pull signals, a recording/reproducing apparatus, and an optical disc.
2. Description of the Related Art
Nowadays, research and development of an optical disc recording/reproducing apparatus, configured for reproducing data recorded on the optical disc or recording data on the optical disc, are proceeding briskly. With such recording/reproducing apparatus, tracking servo is applied using a one-beam push-pull method or a three-beam method. In view of the increasing capacity and recording density of the optical disc, LD laser power or adjustment capability of the optical system, it is desirable to apply tracking servo by the push-pull method.
In the optical disc recording/reproducing apparatus, employing the push-pull method, an output light difference of two photodetectors arranged symmetrically with respect to the track center, that is the output difference between light beams reflected and diffracted by a groove in the optical disc, is detected as a push-pull signal. Tracking servo is applied to, for example, a biaxial actuator, with this push-pull signal as a tracking error.
However, since the optical disc is rotated with a sizeable offset, the light beam spot is moved on the photodetector adapted for detecting the push-pull signal, so that an offset is produced in the push-pull signal, that is the tracking error signal, and hence high tracking accuracy cannot be achieved.
If assumed that the rotating optical disc is not subjected to offset, the reflected laser light is illuminated on the center of a photodetector 50, as shown in FIG. 1A. However, if the rotating optical disc is subjected to offset, the reflected laser light is illuminated at a position offset from the center of the optical disc in dependence upon the disc offset, as shown in FIGS. 1B, 1C and 1D. This produces the dc offset in the push-pull signal. The push-pull signal obtained by the reflected light from the groove is offset relative to an ideal push-pull signal indicated by a broken line in FIG. 2A.
In an optical disc designed to record data on both the land and the groove, offset in the push-pull signal occurs at both the land and the groove, in which case the push-pull signals obtained by the reflected light from the land is subjected to offset from the ideal push-pull signal indicated by a broken line shown in FIG. 2B.
If an offset is produced in the push-pull signal in this manner, correct tracking cannot be realized. Since the interstice between the land and the groove is continually being narrowed to keep up with the increased capacity and recording density of the optical disc, it is desired to achieve more accurate tracking.